Triangular wave generating circuits are used in various applications. One common application is for converting an analog audio signal into a pulse signal in a Class-D power amplifier. One such Class-D Audio Amplifier is described in U.S. Pat. No. 6,791,405 to Tsuji et al., entitled “Triangular Wave Generating Circuit Used in Class-D Amplifier” (the “'405 Patent”), the entirety of which is hereby incorporated by reference herein.
FIG. 1 is a circuit diagram illustrating a prior art circuit 10 for forming a triangular wave (Vout) from a square wave signal. The performance of this triangular wave can seriously influence the accuracy of applications that utilize the triangular wave, such as pulse width modulation (PWM) applications. The switching frequency fSW of the output triangular wave is equal to 1/(Tup+Tdown) wherein Tup is the period of the rise of the triangular wave from VL to VH and Tdown is the period of the falling of the triangular wave from VH to VL. The “up” period Tup is equal to C*Vtri,pp/Icharge, where C is the capacitor C across the operational amplifier 12, Vtri,pp is the voltage difference between VH and VL, and Icharge is the charging current from current source I1. Similarly, the “down” period Tdown is equal to C*Vtri,pp/Idischarge, where Idischarge is the discharging current from current source I2 in FIG. 1. Assuming Icharge is matched to Idischarge, then fSW is equal to Icharge/(2*C*Vtri,pp). From this equation, it is known that the switching frequency of the triangular wave is directly proportional to the charge and discharge currents, Icharge and Idischarge, and inversely proportional to the triangular wave swing (Vtri,pp).
FIG. 2 illustrates potential problems with the triangular wave generators such as the generator 10 of FIG. 1. For example, as shown in “Problem 1” of FIG. 2, the triangular wave does not vary between the desired peak limits VH and VL if the current sources are not matched, i.e., if current source I2>I1 or current source I1<I2. Similarly, “Problem 2” illustrates that this same issue arises if the square wave signal does not have an ideal duty cycle. The second problem is frequently found when the internal clock pulse is not synchronized to an external clock source. Synchronizing an internal clock to an external clock is important in, for example, multiple class D amplifier applications, such a 5.1 channel or 7.1 channel audio systems. If the switching frequency is not the same, a beat frequency will occur in the audio band.
The '405 Patent discussed above teaches a method of providing synchronization with an external clock to form a triangular wave. The method of the '405 Patent achieves synchronization between and internal clock signal and an externally provided clock signal by varying the triangular wave slope, i.e., by varying the charge/discharge currents. However, in class-D amplifiers, the audio quality is affected by the slope of the triangular wave. Once the slope of the triangular wave becomes smaller than the slope of the amplifier's integrator output, the audio quality becomes worse.
Improved triangular wave generators are desired.